Given the looming concerns of climate change, sustainable energy resources such as solar and wind have entered the global spotlight, triggering the search for reliable, low cost electrochemical energy storage. Among the various options, lithium ion batteries are currently the most attractive candidates due to their high energy density, and foothold in the marketplace. However, many factors (cost, safety, and lifetime) will likely limit their large scale applications, and dictate against their use in stationary grid storage where low cost and durability are more of a concern than weight. What is needed is a high energy density battery that is rechargeable, cheap, safe, and easy to manufacture and dispose of or recycle. Aqueous batteries (water based electrolytes) are therefore attracting tremendous attention. Their high conductivity (up to 1 Siemens (S) cm−1) compared to the non-aqueous electrolytes (0.001 to 0.01 S cm−1) also favour high rate capabilities suitable for emerging applications.
The use of metallic negative electrodes is a means to achieve high energy density and ease of battery assembly (hence lower cost). There is a trade-off between the reduction potential of a metal, E°, (low values give higher cell voltages) and safety. Metals with low reduction potentials (e.g., lithium, potassium, calcium, sodium, and magnesium) react with water to produce hydrogen. However, zinc is stable in water and for that reason it has been used as the negative electrode in primary aqueous battery systems. Moreover, zinc has (a) high abundance and large production which makes it inexpensive; (b) non-toxicity; (c) low redox potential (−0.76 V vs. standard hydrogen electrode (SHE)) compared to other negative electrode materials used in aqueous batteries: and (d) stability in water due to a high overpotential for hydrogen evolution. The latter renders a large voltage window (˜2 V) for aqueous zinc-ion batteries (AZIBs) employing a metallic Zn negative electrode.
Vanadium and molybdenum are low cost metals possessing a range of oxidation states (V: +2 to +5; Mo: +2 to +6), which allows for multiple redox and hence large specific capacities for vanadium or molybdenum based electrode materials. Layered VnOm (vanadium oxides: V2O5, V3O8, V4O11) and MoOy (molybdenum oxides) that are made of two dimensional sheet structures were the subject of much past investigation for non-aqueous and aqueous alkali (Li and Na) ion batteries. The additional presence of interlayer neutral molecules, ions, metal ions and/or water of hydration in such layered oxides act as pillars, providing structural stability during long term charge discharge cycling.